The 19-year-old Lewis was found to have an elevated level of Salbutamol, contained in the asthma medication Ventolin, following a doping test in February.
Salbutamol is a specified substance on the World Anti-Doping Agency's prohibited list.
Lewis, who admitted the charge, told the Sports Tribunal of New Zealand at a hearing earlier this month that he had not been managing his asthma and had taken 10 puffs of his Ventolin inhaler in the 24 hours before a game in Dunedin.
"All I was told in relation to being an asthmatic was to be careful and make sure I manage it well and mention it when tested," Lewis told the hearing in the judgment made public today.
"I was never told that I could have too much or that there was a limit."
Lewis, who has played three times for the All Whites and was a member of New Zealand's under-20 World Cup squad last year, was dropped from the senior side before the Oceania Nations Cup in late May-early June this year because of the issue.
DFSNZ and New Zealand Football (NZF) agreed there was no significant fault on Lewis's behalf and the drug testing agency asked for a penalty at the low end of the range.
He was suspended for one month, which ends on August 8.
"I was pretty relieved," Lewis said in a NZF interview. "I was fortunate to get the lowest outcome from the tribunal. Lesson learned.
"I should be managing my asthma a lot better."